In recent years, the service which distributes contents, such as image and music, using a network has already started.
This kind of service requires to distribute contents in real time for a user's request, that is, real-time nature. For this reason, the server which distributes the contents stored in a storage device also requires a high speed and stable performance in reading the contents from the storage device and writing the contents to the storage device.
In these days, from a viewpoint of a high speed and stability, a solid-state drive (it is called “SSD” hereinafter) which uses a flash memory may be used for the storage device.
The SSD has a feature of a fast average data access speed and a low power consumption operation as compared with a hard disk drive (it is called “HDD” hereinafter). On the other hand, as for the SSD in which writing and erasing were repeated and which has been used for a long period of time, it is known that a writing speed will become slow compared with a writing speed of first data writing.
This is because a wear leveling (equalization of writing) and a garbage collection (releasing of an unnecessary data area) are performed as internal processing of the SSD.
When reading continuous data of image, music, etc. from the SSD, if writing of another data is simultaneously performed, the data may be unable to be read stably.
That is, when writing of a certain data takes time, reading of another data may be kept waiting in the meantime. This is not desirable when taking out a continuous data of image, music, etc. In order to reduce this influence, there is a method of reading and writing a data stably in appearance, by having a cache memory in an inside of the SSD.
However, when performing a writing of a data in the midst of read-out of a large amount of continuous data, a bigger cache memory must be prepared for an inside of the SSD by customization. Customization of the SSD is not preferred in respect of cost. It is desirable to use a general-purpose SSD, if possible.
When using the general-purpose SSD for a storage device, since a controller inside the SSD may perform the wear leveling and the garbage collection autonomously, a delay problem peculiar to the SSD will certainly occur.
In addition, since delay of a reading and writing within a definite period of time in a storage device is not treated as a fault, it is not complemented with a redundant data. This is very inconvenient to take out a data continuously in video delivery etc.